Method of uniform secondary recovery



Dec. 10, 1963 J. N. DEW ETAL 3,113,616

METHOD OF UNIFORM SECONDARY RECOVERY Filed March 9, 1960 2 Sheets-Sheet1 INVENTORS JOHN M DEW y FRA N675 R GOA/LE) A TTORNE Y Dec. 10, 1963 J.N. DEW ETAL METHOD OF UNIFORM SECONDARY RECOVERY 2 Sheets-Sheet 2 FiledMarch 9, 1960 INVENTURS F/ 6 JOHN IV. DEW

BY FRANCIS R. CON/.EY

ATTOR 5 United States Patent 3,113,616 METHOD 6F UNEFGRM SECGNDARYRECQVERY John N. Dew and Francis R. (Ionley, Ponca City, Ulda, assiguorsto Continental Oil Company, Ponca City, Okla, a corporation of DelawareFiled Mar. 9, 1969, Ser. No. 13,763 11 Claims. (Cl. 165-9) The presentinvention relates to the recovery of hydrocarbons from partiallydepleted oil fields and pertains more particularly to a new and improvedmethod for applying a combination flooding approach to a fieldwideoperation in order to achieve the ultimate in second ary recovery ofhydrocarbons contained therein.

Oil exists in the sands or similar strata within a hydrocarbon-bearinreservoir in two distinct states, i.e., as free oil that is locatedbetween the voids of the sands and as fixed oil which is held byabsorption and perhaps to some degree by adsorption on the particles ofsand which is commonly referred to as the film of oil that adheres tosand or the particles of the oleiferous structure. Such fixed oilreferred to above can be said to be held by sorption. Generally, oilexists in such formations together with the natural gas, and this gas isnormally under sufficient pressure to force a portion of the free oil tothe surface through a well bore.

In addition to recovering such free oil by utilizing the naturallyoccurring energy of the gas present within the formation, the free oilcan be recovered by creating an artificial fluid or gaseous pressurewithin the formation or by the more conventional method of pumping. Thefixed oil cannot be recovered by the conventional methods used toproduce oil, and it becomes economically difficult to recover such oilby fluid or gaseous pressure. In some cases the total amount of oilremaining in the formation after pumping has become unprofitable canequal 60 to 90 percent of the original oil in place. Heretofore, variousmethods have been proposed in an attempt to recover a portion of theso-called residue oil. In one well known method water is introducedunder pressure into a number of well bores, and the oil within theformation is then produced with a portion of the water at other recoverywells by the resultant pressure of the water drive. The use of Water asa flooding medium is oftentimes quite limited, and many variations havebeen attempted in order to convert the water to a more eflicientdisplacing medium, such as the addition of surface-active agents.Natural gas and air have also been used in a similar fashion to thewater flood, but due to various reasons or circumstances these methodsof secondary recovery have not been as satisfactory as desired.

Another method recently developed for the secondary recovery of oil isgenerally referred to as in situ combustion. in this method an oxygencontaining gas, such as air, is ordinarily injected into the well boreand formation through certain designated wells in order to create acombustible mixture within the reservoir. Thereafter the well bore issubjected to heat treatment, and the formation surrounding the well boreis heated sufliciently to cause the ignition of the hydrocarbonscontained therein. The secondary recovery is accomplished by themovement of the displacing heat front through the reservoir, and thehydrocarbons therein are produced at an appropriate recovery well. Thismethod has several disadvantages, such as equipment limitations,formation limitations, geological limitations, and the inherent problemsof channeling.

The most recent attempt to recover oil from partially depletedreservoirs has been through the utilization of a method generallyreferred to as miscible flooding. This approach utilizes the injectionof fluid which are miscible with the hydrocarbons within the formationand have a high displacement efliciency. The drawback of this type offlooding operation is that the sweep efiiciency, While very good incertain types of formations, is generally considered to be quite poor sothat volumetric displacement of oil is economically unfeasible in thenormal situation.

Each of these types of approaches to the secondary recovery of oil frompartially depleted reservoirs has individual limitations andrestrictions, making it quite difiicult to develop and apply aneconomical method of secondary recovery.

The principal object of the present invention is to provide a method forthe secondary recovery of oil from partially depleted reservoirs whichavoids the disadvantages inherent in the prior art methods.

A further object of our invention is to provide a method for thesecondary recovery of oil from a partially depleted reservoir whichachieves the ultimate in economic recovery of the oil present therein.

These and additional object-s and advantages of the present inventionwill be apparent to persons skilled in the art on reading the followingdescription and drawings which disclose our invention.

Broadly the present invention is a method for the recovery of oil from apartially depleted oil-bearing reservoir by spacing the Wells in orderto take advantage of the inherent characteristics of the fluid type offlooding medium and achieve the ultimate recovery possible. The methodis to inject successive fluids into the reservoir to be flooded in apredetermined field-wide pattern, the first fluid to be injected willpreferably have a low viscosity and therefore have the greatest tendencyto finger from the injection well to the recovery well. The overallmethod disclosed herein anticipates the utilization of the fingeringtendency; and to encourage the same, alternately established recoverywells should be opened and the pressure drawn down, thereby causing theacute fingering to occur even more rapidly. Subsequent tothebreakthrough at the initial recovery well, the flooding media are theninjected into both the initial injection and the initial recovery wells,thereby establishing a uniform zone of displacing fluid Within thereservoir. Subsequently, displacing fluids of equal or greater viscositythan the viscosity of the initial displacing fluid are then injectedinto the alternate well bores, thereby displacing thepreviously-injected displacing fluid and incorporating into thereservoir a selectively placed zone of eflicient displacing fluid havinga most uniform front which is then prepared for diversion anddisplacement throughout the formation. The ultimate recovery fluid whichcan also be of viscosity greater than that of the initial injected fluidis then injected into all of the injection Wells, and the displacingfront moves outwardly into the formation displacing the oil thereintoward the recovery wells wher in it is withdrawn to the surface andproduction is accomp-lished.

In the drawings:

FIGURES 1 through 3 represent the step-wise application of thisinvention to the well pattern for a straight line drive flood.

FIGURES 4 through 6 represent the application of this invention to aseries of five-sp=ot pattern well arrangemerits.

FIGURE 1 shows a line pattern of 15 wells which is susceptible to theapplication of the present invention. Reference numerals 1 through 15are drilled wells, with numerals l, 3, 5, ll, 13, and 15 beingdesignated as initial injection wells for the initial injection phase ofthis method and wells represented by numerals 2, 4, 12., and 14 beinginitial recovery wells for use during initial injection phase. Theshaded areas designated by reference numerals I6 and 17 represent theinitial displacement zones created by the initial injection and recoveryas developed in compliance with the method disclosed herein.

FIGURE 2 is a representation of the line drive arrangement of 15 wellsreferred to above with zones of secondary displacement designated asnumerals 18 and 19 which include zones 16 and 17. These zones ofsecondary displacement attain increased size by the conversion of wellsrepresented by numerals 2, 4, l2, and 14 to secondary injection wells sothat during the secondary injection stage wells 1 through 5 and 11through are all secondary injection wells.

FIGURE 3 depicts the application of the present method of secondaryrecovery to the line pattern of wells for the ultimate displacement fromthe formation. The wells represented by numerals 6 through Ill are theultimate recovery wells, and the injection of the displacing fluids ofwells 1 through 5 and 11 through 15 is the ultimate phase of recovery.The zones 18 and 19, established by the initial and secondary injectionefforts, create a uniform line drive zone which, when displacing fluidsare injected in the third or final recovery phase through ultimateinjection through wells 1 through 5 and 11 through 7.5, allows theutilization of a previously formed uniform fluid front. The uniformdisplacement fronts bounding zones 18 and I? move inwardly toward theline of ultimate recovery wells designated as numerals 6 through It InFIGURE 3, the numeral 20 represents the ultimate displacement zone whichis effectively swept during the ultimate injection and recovery step bythe method of secondary recovery disclosed in the present application.

FIGURE 4 discloses an overall field layout consisting of groups of wellsin patterns generally referred to as inverted five-spot well patterns.Specifically these patterns are represented by numerals 21 through 25;25 through 29; 2? through 33; 24, and 33 through 36; and 24, 25, 29, 33,and 37. The first phase of initial displacement is accomplished byinjecting displacement fluid into initial injection wells represented bynumerals 23, 28, 32, and 36, with the initial recovery wells beingrepresented by numerals 21, 22, 24 through 27, 29 through 31, and 33through 35. The initial displacement zones, represented by numerals 38through 41, are the zones created by the first phase of displacement inthe present method of secondary recovery. This figure could representalso non-inverted five-spot patterns by making the opposite wellsinjection and recovery wells.

FIGURE 5 represents the second phase of secondary recovery as disclosedin the method herein, and the Wells contained therein are identical tothose disclosed in FIG- URE 4. The secondary displacing fluid isinjected into all of the initial injection wells 23, 28, 32 and 36, andall of the initial output wells as set forth in the description ofFIGURE 4 are converted to secondary injection wells, which includeswells 21, 22, 24- through 27, 29 through 31, and 33 through 35. Thezones represented by numerals 42 through 45 are the zones of secondarydisplacement which are created by injecting into the secondary injectionwells. The secondary displacement zones 42 through 45 include theinitial displacement zones and the boundaries of said zones provide theuniform flood front for the ultimate recovery phase of the method.

FIGURES 6 shows the application of the method of secondary recovery asdisclosed in the present application as it is applied to the zonesurrounding the ultimate output well 37. The shaded zone 46 representsthe zone of the formation which is subjected to the uniform displacementfront above for the ultimate recovery effort of the present inventionand from which all of the oil is displaced through the ultimate outputwell 37.

A particular embodiment or operation of the present invention as shownin FIGURES 1 through 3, is described in order that the invention will bereadily understood. The method disclosed herein is most successful .41when applied on a field-wide operation utilizing a plurality of wellswithin the field, but it may be applied to a specific area whichcontains oil that has eluded previous recovery attempts. The first stepof this method is the selection of an initial well pattern to besubjected to this method of secondary recovery, which is based upon thedetermination that the pattern selected is of the proper size, shape,location, and susceptibility to production. This step requires the wellsbe spaced, either by utilizing existing wells or drilling others, toaffirmatively establish the wells in a pattern in order to flood theformation in a predetermined size, shape, and location. The patternwhich is selected can be of any type available within the particularfield as the situation allows and can be a line pattern, as in FIGURES 1through 3, a five-spot pattern, or a special well pattern which isavailable within a particular field.

The method of the present invention as illustrated in FIGURES 1 through3 requires that a pattern be selected to include wells which will beadaptable as initial injection wells (1, 3, 5, 11, I3 and 15) andinitial recovery wells (2, 4, 12 and 14) during the initial displacementstep to create the desired initial displacement zones or manifolds (16and 17) with the initial recovery wells being convertible to injectionwells for the secondary injection step. These wells represent only theinitial operational Wells in considering the complete pattern, andshould be located so that the ensuing arcs of the intersecting lines ofthe subsequently developed secondary displacement zone or manifoldboundaries are opposite the recovery wells to be used during theultimate recovery phase. The arcs of the lines of intersection, definingthe outer limits of the zone or manifold are therefore positioned sothat the ultimate recovery wells (6 through 1(3) are located in theadjacent areas surrounding the initial and secondary injectionoperational pattern. The positioning of the wells is dependent upon thearcs of the lines of intersection, which are the outer limits of thesecondary displacement zones, and the criticality varies according tothe degree of acuteness of the arcs. As the arc approaches a straightline, the ultimate recovery wells can be located in any positionopposite the zone as shown in FIGURES 1 through 3.

The distance of the ultimate recovery wells from the secondarydisplacement zones affects the economics of the injection equipment andthe amount of energy lost during the displacement, so the wells cannotbe too distant and achieve optimum economical production. The initialwell pattern selected should preferably be located within the interiorarea of a field to allow subsequent expansion of the recovery method tosimilar adjacent patterns in order that the entire volume of the fieldcan be subjected to the secondary recovery efforts. Directionalpermeabilities should also be taken into consideration in well patternselection and can be determined by small scale preliminary air or gasinjection tests to determine the direction of such flow in the specificreservoir, or through the use or laboratory measurements on cores whosedirectional orientations have been obtained during the coringoperations.

The second step, after selection of the pattern wells, is the initialinjection phase which is conducted by conventional means known in theart within the formation immediately surrounding the initial injectionwells. The displacement media injected into the hydrocarbon-bearingformation will be governed by the characteristics of the hydrocarbonswithin the individual formation to which this method of secondaryrecovery is to be applied and the availability of supply. In operationit is desirable to initially inject into the formation through theinitial injection wells, a displacement fluid which has a low viscosityrelative to the hydrocarbons within the formation. The viscosity ratiobetween the hydrocarbon contained within the formation and the initialdisplacing fluid should be preferably in the order of a factor of 4 orgreater. The actual controlling feature being that the displacing fluidshould be of such a viscosity that it will finger or channel directlyinto the initial recovery wells with rapidity. The fingering which isstimulated by the injection of such a displacement fluid, coupled withthe reduction in pressure of the initial output wells, will enable theordinarily undesirable fingering efiect to be utilized in achievingimproved displacement zones.

The displacing fluids which can be used during the ini tial injectionare gas, liquid hydrocarbons, or ot 161 liquids known in the art,dependent upon the characteristics of the formation hydrocarbons asindicated above. The preferable initial injection fluid is one whichwould be of the proper viscosity and miscible with the hydro carbons ofthe formation, such as liquefied, low-molecular weight, normally gaseoushydrocarbons, generally referred to as liquefied petroleum gases, whichinclude methane, ethane, propane, butane, pentane, hexane, and heptane.The initially injected displacement fluid will displace substantiallyall of the hydrocarbons within the formation, and will establish aninitial displacement zone within the formation which is the basis forthe uniform driving mechanism of the overall secondary recovery method.The initial injection is made through wells 1', 3, 5, 11, i3, and 15, inorder to displace the hydrocarbons to the initial recovery wells withrapidity.

The initial recovery wells are 2, 4, 12 and 14 and are maintained in acondition of reduced pressure during this second step to stimulate thefingering or channeling effect to the utmost until production issubstantially the injected displacement fluid. The ultimate recoverywells, FTG. 1, nuznerals 6 through are shut-in, back pressured, ordrilled later as needed to prevent production of the initial displacingfluid therefrom during the initial injection phase. The initialinjection phase is conducted as rapidly as possible until the initialdisplacement zones, 16 and 17, have progressed through the areasurrounding the initial recovery wells, and break through which causesthe initial displacement zones to intersect about the initial recoveryWells. This initial injection step creates normal recovery drives andoil is produced accordingly from the initial recovery wells to be a partof the production realized from the present invention.

The third step of the present method is to enlarge the initialdisplacement zones about the initial injection and recovery wells by thesecondary injection phase of the method. The initial recovery wells 2,4, 12 and 14, are converted to secondary injection wells after thedisplacement fluid has broken into them as initial recovery wells, inorder to establish uniform secondzuy displacement zones 18 and 19. Thenormally undesirable channeling effect developed by the initialdisplacement zone is thereby utilized to create the desired manifold oftie secondary displacement zone within the limitations of the lines ofintersection. These initial and secondary injection phases form thesecondary displacement zone of increased permeability due to therecovery therefrom of the fluid containing hydrocarbon deposits whichwould otherwise impede the movement of fluids therein, and effectivelyestablish an underground manifold system within the formation.

The secondary displacement fluid injected into the formation during thesecondary injection phase is injected into the secondary injectionwells, and can be any known displacement fluid which is available forutilization. The method disclosed herein anticipates the use of adisplacement media in the secondary displacement phase having aviscosity that is equal to or greater than the viscosity of the initialdisplacement fluid. Any of the liquefied etroleum gases set forth aboveor more economical liquid fluids, such as Water, can be utilized. Duringthe secondary injection phase all of the initial injection and recoverywells are converted to secondary injection wells as the situationnecessitates, and the secondary injection phase is conducted untilsufficient amounts of secondary displacement fluid are injected in orderthat uniform secondary displacement zones, 18 and 19, of sufficientboundaries have been established within the formation.

The ultimate phase of this method is commenced after the termination ofthe secondary injection phase which creates the limits for the uniformflood fronts. The initial injection wells and the initial output wells,converted to secondary injection wells, or any number thereof, areconverted to ultimate injection wells 1 through 5 and 11 through 15, forthe ultimate recovery phase of the present method of hydrocarbonrecovery. The ultimate recovery wells 6 through It) can be drilled asneeded to serve as recovery wells during the ultimate recovery phasewhich can be performed using the former injection and recovery wells ofthe injection steps as ultimate injection wells. The secondarydisplacement zones or manifolds within the subterranean formation aresubjected to the injection of flooding media through all or any portionof the wells within said zones in suflicient amounts to fill the entirezones of the formation with the desired displacing media to thepreviously developed limits as determined by the foregoing injectionphases. The injection of the displacement media is continued after thesecondary displacement zones are filled to establish a uniform ultimatedisplacement drive which thereupon becomes an effective uniformdisplacement vehicle for the recovery method.

The ultimate step of the recovery method disclosed herein is theinjection of a displacement fluid through the secondary injection wellsmade available by either one of the preceding steps hereinafter referredto as ultimate injection wells. The ultimate recovery phase willnormally involve the recovery of the hydrocarbons from the largest zoneof the formation, and it therefore becomes necessary to consider theeconomics of the flooding media to be used in the operation. Althoughany of the known displacement fluids are satisfactory for the ultimaterecovery of oil from the formation, it is economically advantageous touse an efficient medium which is relatively inexpensive per unit volume,due to the large amount necessary. The ultimate fluid injected into theformation to displace the hydrocarbons to the ultimate recovery wellsshould have a viscosity equal to or greater than the viscosity of thesecondary displacement fluid, preferably water, due to the fact that itis inexpensive and normally available. Water is susceptible of beinginjected into the formation whereupon it displaces substantially all ofthe hydrocarbons from within the ultimate displacement zones of theformation to the ultimate recovery wells where it is transferred to thesurface by means known in the art.

During this phase the secondary displacement zones, 18 and 19, whichhave been created by this stepwise procedure, act as a vehicle to causea controlled directional drive to be established in focusing the uniformfluid drive toward the adjacent ultimate recovery wells, 6 through 10,now opened to the ultimate recovery of hydrocarbons. The shapedboundaries of the zones or manifolds in the form of arcs, cause injectedmedia to be directed linearly toward the ultimate recovery wells at auniform and regular rate in a previously determined direction to createthe ultimate recovery zone, 28, as shown in FIGURE 3.

When water is used for the ultimate phase of the method set forth as thepresent invention, the ultimate recovery zone of the formation which hasbeen finally subjected to water flood is substantially filled with saidwater. This water is an efficient back-fill from a functional andeconomic viewpoint, which is favorable because the water is essentiallyan incompressible medium. It acts as an eificient fill when subjected tosubsequent gas, oil or Water pressure as would occur when adjacent areasof the field are subjected to subsequent secondary recovery efforts. Thewater is also an inexpensive me dium and can be injected by pumping atless cost than other materials, such as a gas. The Water deposited inthe formation during displacement, as a back-fill, precludes thenecessity of maintaining air or gas injection units at the site afterthe secondary recovery is concluded, thereby reducing the amount ofinvestment ordinarily necessary otherwise.

Another embodiment of this invention is set forth in FIGURES 4 through 6which represent the application of the method to five-spot floodingpatterns. The method would be essentially identical to the abovedetailed description of the operation, but applied to five-spotpatterns.

In order to disclose still more clearly the nature of the presentinvention and the advantages thereof, reference will hereinafter be madeto certain specific embodiments which illustrate the flexibility of theprocess. It should be clearly understood, however, that this is donesolely by way of example and not to be construed as a limitation uponthe spirit and scope of the appended claims.

Examples The oil sand formation within a field is at a depth fromapproximately 1230 feet to approximately 1255 feet, and averaged 16percent porosity with a horizontal permeability of 95 millidarcies. Anaverage of 50 percent of the pore space is filled with oil, 40 percentwith water, and the remainder with gas. The viscosity of the reservoircrude at the formation temperature of 81 F. is 18 centipoises. A portionof the oil sand having a thickness of 25 feet is readily capable ofbeing flooded by means of a straight line drive flood pattern. Thiswould utilize three rows of wells with each row containing five wells,such as illustrated in FIGURE 1 of the drawings. The wells are spaced660 feet apart on a 10-acre spacing pattern.

The first step of the recovery process is the injection of propane intothe alternate wells of the two outside rows of wells. A propane having aviscosity of approximately 0.10 centipoise at the reservoir temperatureof 81 F. is injected into the formation through the initial injectionwells until an average of 8,000 barrels of propane is injected into eachwell. The sand face injection pressure average about 900 p.s.i., whilethe sand face pressure in the alternate initial output wells isessentially psi.

A breakthrough of propane into the other alternate wells, initialrecovery wells, occurs after the above amount of propane is injectedinto each well as above. After propane breakthrough occurs into allinitial recovery wells, the initial oil recovery wells are convertedinto secondary injection wells, and propane injection is continued untila total of 8,000 additional barrels of propane is injected.

At this time water injection is initiated into all of the previousinjection wells, and the ultimate output wells are opened to production.Water injection is continued until oil recovery drops below the economiclimit, and approximately 80 percent of the oil sand is subjected todisplacement by following this procedure whereby an average residual oilsaturation of 15 percent of the pore volume is attained.

In another part of the same oil field, a portion of the oil sand isflooded most readily by using a '-spot well pattern similar to thatshown in FIGURE 4 of the drawings. Since the original spacing of thewells is l0-acre spacing, drilling is conducted between the originalrows to furnish a row of wells staggered to create the desired 5-spotpattern as above. Into the center wells, initial injection wells,propane is injected until propane breakthrough occurs in the foursurrounding wells of each of the five-spot pattern. Approximately 12,000barrels of propane are injected through each initial injection well inorder to achieve the breakthrough. As this point Water injection isstarted into both the initial injection wells and the initial recoverywells, and continued until approximately 10,000 barrels of water areinjected to enlarge the displacement zone. Then, the ultimate recoverywells are opened with the injection of water continued, whereby oilrecovery is initiated from the ultimate recovery Wells.

o The method disclosed causes displacement of the oil from approximatelypercent of the oil sand, whereby a residual oil saturation of 15 percentis attained.

While particular embodiments of the invention have been described, itwill be understood, of course, that the invention is not limited theretosince many modifications may be made, and it is contemplated to cover bythe appended claims any such modifications as fall within the true scopeand spirit of the invention.

The invention having thus been described, what is claimed and desired tobe secured by Letters Patent is:

l. A method for the recovery of hydrocarbons from a subterraneanformation traversed by a plurality of Well bores whichcomprisesinjecting an initial displacement fluid into a first group ofsaid well bores to displace the hydrocarbons through the formation to asecond group of said well bores located with respect to said first groupso as to define an elongated pattern; withdrawing said hydrocarbonsthrough said second group of well bores to the surface to define anelongated displacement zone of increased permeability communicatingbetween said first and said second group within the reservoir; injectinga secondary displacement fluid into substantially all of said first andsecond groups of well bores while maintaining the remaining well bores,located substantially normal to said elongated pattern, substantially\at a pressure at least as high as atmospheric pressure to uniformlyenlarge said elongated displacement zone around said first and secondgroups of well bores; injecting into substantially all of said first andsecond groups of well bores an ultimate displacement fluid outwardlyfrom the thus-produced uniform enlarged elongated zone to displace thehydrocarbons through said formation to said remaining well bores; andwithdrawing said hydrocarbons through said remaining well bores to thesurface.

2. The method defined in claim 1 wherein the first group of well borescomprises the outer members of a plurality of five spot patterns whichare situated around at least one centrally located well bore and whereinthe second group of well bores comprises the central well bores of eachfive spot pattern.

3. The method defined in claim 1 wherein the first group of well borescomprises the central members of a plurality of five spot patterns whichare situated around at least one centrally located Well bore and whereinthe second group of well bores comprises the outer members of each fivespot pattern.

4. A method as set forth in claim 1 wherein the initial displacementfluid is liquefied, low-molecular-weight, normally-gaseous hydrocarbons.

5. A method as set forth in claim 4 wherein the secondary displacementfluid is water.

6. A method as set forth in claim 4 wherein the ultimate displacementfluid is water.

7. A method as set forth in claim 1 wherein the initial displacementfluid is miscible with the formation hydrocarbons.

8. A method for the recovery of hydrocarbons from a subterraneanformation traversed by a plurality of well bores which comprisesinjecting an initial displacement fluid of low viscosity into a firstgroup of said well bores to displace the hydrocarbons through theformation to a second group of said well bores located with respect tosaid first group so as to define an elongated pattern; withdrawing saidhydrocarbons through said second well bores to the surface to define anelongated displacement zone of increased permeability communicatingbetween said first and said second group; injecting a secondarydisplacement fluid of viscosity at least as high as that of said initialfluid into substantially all of said first and second groups of wellbores While maintaining the remaining well bores, located substantiallynormal to said elongated pattern, substantially at a pressure at leastas high as atmospheric pressure to uniformly enlarge said elongateddisplacement zone; injecting into substantially all of said first andsecond groups of well bores an ultimate displacement fluid of viscosityat least as high as the viscosity of the secondary displacement fluidoutwardly from the thus-produced uniformly enlarged elongated zone todisplace the hydrocarbons through said formation to said remaining wellbores; and withdrawing said hydrocarbons through said remaining wellbores to the surface.

9. A method for the recovery of hydrocarbons from a subterraneanformation traversed by a plurality of well bores of injection, initialrecovery, and ultimate recovery wells which comprises spacing theinjection and initial recovery wells to provide an initial displacementzone of predetermined size, shape and position of elongated pattern;injecting an initial displacement fluid of low viscosity through theinjection wells into the formation to displace the formationhydrocarbons toward the initial recovery wells; withdrawing hydrocarbonsfrom the formation through initial recovery wells; terminating theinjection of said initial displacement fluid upon substantialbreakthrough into the initial recovery wells whereby an initialelongated displacement zone of increased permeability is defined;injecting a secondary displacement fluid of viscosity at least as highas that of said initial displacement fluid through the injection and theinitial recovery wells into the initial displacement zone whilemaintaining the remaining well bores substantially at a pressure atleast as high as atmospheric pressure to displace the formationhydrocarbons outwardly from said zone; terminating the injection of saidsecondary displacement fluid after the initial displacement zone isuniformly enlarged to define a secondary elongated displacement zone;spacing said ultimate recovery wells substantially normal to saidsecondary elongated displacement zone; injecting an ultimatedisplacement fluid of viscosity at least as high as that of thesecondary displacement fluid through the injection wells and initialrecovery wells to displace the formation hydrocarbons outwardly fromsaid secondary displacement zone toward the ultimate recovery wells;withdrawing formation hydrocarbons through ultimate recovery wells tothe surface; and terminating the injection of displacing fluid.

10. The method for recovering connate fluid from a subterraneanformation traversed by a plurality of well bores which comprises thesteps of injecting a fluid into at least a first of said bores whilewithdrawing connate fluid from at least a second of said bores so as toremove a portion of said connate fluid from said formation in an areaaround a plurality of the well bores located so as to establish anelongated zone of increased permeability within said area communicatingbetween said plurality of the well bores; injecting a fluid underpressure by way of at least said firs-t and second bores into said areawhile maintaining pressure in well bores adjacent said area to enlargesaid area; and removing connate fluid from an adjacent well bore locatedsubstantially normal to said elongated zone while injecting additionalfluid by way of at least said first and second bores into said area.

11. The method for recovering connate fluids from a subterraneanformation which comprises the steps of drilling a plurality of wellsinto said formation, said wells being arranged into a first group ofrows and a second group of rows the individual rows of which alternatewith the individual rows of the first group; injecting a fluid intoalternate wells in said first group of rows to displace connate fluid insaid formation toward the remaining wells of said first group whilewithdrawing connate fluid from the wells remaining in said first groupso as to establish an elongated zone of increased permeability betweensaid alternate wells and said remaining wells of said first group;stopping injection upon breakthrough of injected fluid in said wellswherein said connate fluid is being Withdrawn; injecting fluid insubstantially all Wells in said first group while maintaining the wellsin said second group closed to form an enlarged elongated pressuremanifold in said formation; and withdrawing connate fluid from saidformation by way of at least a portion of the wells in said second groupwhile injecting a drive fluid into at least a portion of the wells insaid first group.

References Cited in the file of this patent UNITED STATES PATENTS2,347,778 Heath May 2, 1944 2,798,556 Binder et al. July 9, 19572,885,002 Jenks May 5, 1959 UNITED STATES PATENT OFFICE CERTIFICATE OFCORRECTION Patent No. 3,113,616 December 10, 1963 John N. Dew et a1.

It is hereby certified that error a ppears in the above numbered patentrequiring correction and that the said Letters Patent should read ascorrected below.

Column 1, line 72, for "fluid" read fluids column 3, line 64, for"FIGURES" read FIGURE column 5, line 58, after "the", second occurrence,insert initial displacing column 7, line 41, for "average" read averagesline 69, for "As" read At Signed and sealed this 2nd day of June 1964.

(SEAL) Attest:

ERNEST W. SWIDER EDWARD J. BRENNER Awasting Officer Commissioner ofPatents

10. THE METHOD FOR RECOVERING CONNATE FLUID FROM A SUBTERRANEANFORMATION TRAVERSED BY A PLURALITY OF WELL BORES WHICH COMPRISES THESTEPS OF INJECTING A FLUID INTO AT LEAST A FIRST OF SAID BORES WHILEWITHDRAWING CONNATE FLUID FROM AT LEAST A SECOND OF SAID BORES SO AS TOREMOVE A PORTION OF SAID CONNATE FLUID FROM SAID FORMATION IN AN AREAAROUND A PLURALITY OF THE WELL BORES LOCATED SO AS TO ESTABLISH ANELONGATED ZONE TO INCREASED PERMEABILITY WITHIN SAID AREA COMMUNICATINGBETWEEN SAID PLURALITY OF THE